DEVELOPMENT OF SUPRAPERICARDIAL BODY 379 



The earliest appearance of the suprapericardial body on the 

 right side is found in embryos of about 33 mm. in length (S. C. 

 8 and H. E. C. 186). It has about the same corresponding posi- 

 tion as the body on the left side. The relation of the glands to 

 the pharynx and its derivatives at this stage is shown in figure 

 3 which is from a wax reconstruction of the pharynx of an embryo 

 33.1 mm. long (S. C. 8). The arching of the pharynx present in 

 the younger embryos (fig. 2) has almost entirely disappeared. 

 The cavity of the pharynx has become somewhat narrowed in 

 its dorso-ventral diameter because of the rapid growth of the 

 underlying heart. The gill-pouches have a broad attachment 

 to the lateral regions of the pharynx. Each succeeding pair of 

 pouches, the second to sixth inclusive, arises from the pharynx 

 a little lateral to the pair preceding. This condition is more 

 apparent than real, because the ventral extremities of the more 

 caudal pouches are pushed ventrally and laterally by the growth 

 of the heart. The expanded distal portions of the pouches are 

 arched caudally so as to imbricate the succeeding ones. The 

 thyreoid is a shield-shaped mass of tissue located in the median 

 line about on a level with the origin of the second pouch. Rem- 

 nants of its early connection with the pharyngeal wall are found 

 in a thin broad pouch just below the mouth. The thymus is 

 represented by knob-shaped epithelial thickenings on the dorsal 

 extremities of the last five pair of pouches. 



The suprapericardial bodies are medial and slightly caudal to 

 the ventral extremities of the sixth gill-pouches. They have 

 assumed a position somewhat more medial than that in the 

 younger embryos, lying about half way between the sixth pouch 

 and the median line. The right suprapericardial body is much 

 smaller and more rudimentary than the left. It consists of two 

 slender villous-like cords of epithelium connected to the pha- 

 ryngeal epithelium. There is no lumen present in either of the 

 cords. Van Bemmelen describes a similar condition in Acan- 

 thias where he found the gland consisting of three small ducts 

 opening into the pharynx. 



The left suprapericardial body in the 33.1 mm. embryo (S. C. 

 8) is roughly triangular in shape. The connecting-stalk is at- 



